1. Field of the Invention
The invention relates to supression of solar background in laser receivers that utilize atomic resonance transitions as an ultranarrowband optical filter and more particularly to a filter which is based on transitions in calcium and overlaps the g Fraunhofer line at 422.674 nm in the deep blue spectral region.
2. Description of the Prior Art
There is a strong need to develop communication systems to submarines from satellites and aircraft. One such technique is illustrated in FIG. 1. On the signal from a ground station 6, laser transmitter 2 transmits a signal at wavelength .lambda..sub.s. After passing through the atmosphere 8 and seawater 10, signals at wavelength .lambda..sub.s are filtered by atomic resonance filter 12 which blocks out background sunlight .lambda.. Filter 12 converts signals at wavelength .lambda..sub.s to output emission at wavelength .lambda..sub.o. That output is detected by detectors 14 like photomultiplier tubes which are mounted on submarine 16.
The blue-green spectral region is considered the best wavelength region for underwater communication because the maximum penetration of ocean water by electromagnetic radiation occurs here. Therefore, techniques based upon blue-green laser transmitters are being pursued for this application. The feasibility of underwater laser communication hinges upon the development of an ultranarrowband, wide-angle optical filter that can adequately reject solar background. Other important applications include antisubmarine warfare, interplanetary laser communications, laser radar, sea floor mapping, atmospheric radar, plasma diagnostics, and combustion research.
In the past, multi-layer dielectric filters and birefringent filters were developed for this application. In recent years emphasis has shifter to atomic filters. These devices are ultranarrowband (0.01 .ANG.), isotropic, wide field of view filters. They derive their ultranarrowband properties from transitions between discrete atomic levels. Atomic filters operate at numerous discrete wavelengths throughout the near UV, visible and near IR spectral regions. Laboratory exploration of atomic filtering action has been conducted on cesium and rubidium ground state species and optically-pumped transitions in rubidium, thallium-cesium, potassium, and magnesium. A recent report from the Naval Ocean Systems Command (NOSC) provides an assessment of ultranarrowband optical filter performance and developmental status for submarine communications. R. D. Anderson, R. R. James, and J. W. Rockway, NOSC Tech. Rep. 1291 (1989).
U.S. Pat. No. 4,829,597 teaches that the Fraunhofer lines can be exploited to significantly reduce solar background. Fraunhofer lines are intense dips (minima) in the solar spectrum. They arise from absorption in the outer layer of the sun by heavy elements. These lines have been well studied by solar astronomers. At the most intense lines, the solar background falls below 10% of its continuum value. These dips are natural low background channels for narrowband atmospheric optical transmission. A laser communications system operating in a Fraunhofer dip with a suitable ultranarrowband filter would experience significantly reduced background compared to operation against the full solar background.
However, there are fewer than ten intense Fraunhofer lines in the blue-green spectral region. Owing to their narrow spectral width and the extremely sharp widths of atomic transitions (0.01 .ANG.), it is highly improbable that an exact overlap between a Fraunhofer line and a random atomic filter can be found.
U.S. Pat. No. 4,829,597 discloses an atomic filter that overlaps an intense Fraunhofer line at 518 nm. It is based upon transitions in the triplet manifold of magnesium. This filter possesses highly desirable filter properties as well as strong solar background rejection. The details of the Mg filter operation can be found elsewhere. See J. A. Gelbwachs, Laser Spectoscopy VIII, W. Persson and S. Svanberg, eds. (Spring-Verlag 1987) p. 409. It is the first atomic filter that matches an intense Fraunhofer line.
However, the deep blue spectral region (420-450 nm) has been shown to be a preferred band for transmission through seawater. If a good atomic filter could be found in this region it would afford an additional wavelength option for submarine communications, antisubmarine warfare, and seafloor mapping.
Therefore, the principal object of the present invention is to provide a filter which maximize solar background rejection.
It is another object to provide a filter which operates in the deep blue spectral region (420-450 nm), thus providing good penetration through deep seawater.